1. Field of the Invention
This invention relates generally to the transmitting and receiving of analog signals wirelessly through the atmosphere. More particularly, the present invention relates to digitizing the analog signals, compressing the digitized analog signals according to the Moving Pictures Experts Group (MPEG) layer 3 algorithm, wirelessly transmitting and receiving the compressed, digitized analog signals, and restoring the compressed, digitized analog signal to reproduce the analog signals.
2. Description of Related Art
The wireless transmission of analog signals such as audio signals is well known in the art. Current commercially available devices such as cordless telephones and cellular telephones are examples of the wireless transmission and reception of audio frequency analog signals. U.S. Pat. No. 5,832,024 (Schotz et al.) illustrates the wireless transfer of audio frequency analog signals created by devices such as an AM/FM tuner to speakers. This wireless transfer is generally accomplished by modulating a radio frequency (RF) carrier signal with the analog signal.
The Federal Communication Commission (FCC) of the United States (U.S.) government strictly regulates the conditions at which information can be transmitted into the atmosphere. The FCC has allocated under 47 CFR§ 15.249 certain RF bands above 900 Mhz to be unlicensed. These bands generally require transmission of RF signals having less than 1 mW of power for signals with conventional modulation and 1W of power for spread spectrum modulation. U.S. Pat. No. 5,272,525, U.S. Pat. No. 5,410,735, and U.S. Pat. No. 5,666,658, all to Borchardt et al., describe wireless audio systems that operate at frequencies greater than 900 MHz. U.S. Pat. No. 6,212,359 (Knox) describes a transmission system that receives digitized music from a receiver tuner employing the RF frequencies greater than 900 MHz.
The algorithms defined by the MPEG for the compression of digitized analog signals are published as ISO/IEC11172 and ISO/IEC 13818. The algorithm published under ISO/IEC11172 is commonly referred to as MPEG-1 layer 3 and the algorithm published under ISO/IEC 13818 is referred to as MPEG-2 Advanced Audio Coding (MC). These algorithms will be referred to hereinafter as MP-3 and encompass these and any derivative algorithms as developed under the auspices of the MPEG.
For the MP-3 compression of a digitized analog signal, the analog signal is sampled at a frequency of 44.1 kHz. Each sample has 16 bits. Without compression, one second of a stereo audio signal requires more than 1.4M bits of data. The MP-3 algorithm allows compression of the data by a factor 12 for stereo audio having a bandwidth greater than 15 KHz to a factor of 96 for telephone quality audio having a bandwidth about 2.5 KHz.
The MP3 is an open standard having many versions of programs that implement the standard. Further, there are also digital signal processors and microconntrollers that provide economical hardware to execute the programs that implement the algorithms. As the demand for less expensive and more consumer oriented audio systems increased, application specific integrated circuits (ASIC's) were developed to implement the algorithms of the MP3 specifications. Originally, the MP3 standards were intended for use mainly with video coding for digital storage media. However, presently the MP3 standards are used in digital audio broadcasting, telephonic transmission for distribution of broadcast content, archival storage within broadcasting systems, sound for digital television, internet streaming of audio signals (music), for use within portable audio devices containing stored audio content for replay, and for storage and exchange of digitized music files on computers. Each of these applications either store or retain the compressed, digitized analog signals on a storage medium (semiconductor memory or magnetic or optical storage media) or for use in broadcast of the audio signals to a wide area for use by a general population.